1. Field of the Invention
The present invention relates to a disk-mounting structure for an optical encoder.
2. Description of the Related Art
A prior-art disk-mounting structure for an optical encoder is constructed as shown in FIG. 2 and FIG. 3.
In the drawings, 20 denotes a housing which is attached to a bracket of a motor (not illustrated). 21 denotes a shaft which is supported on the housing 20 via a bearing 22, a thinner part 21b is provided on the upper portion of said shaft 21, and a thicker part 21c is provided on the lower part of the same. Also, a step 21a is provided between the thinner part 21b and the thicker part 21c. 23 denotes a rotating disk made of a transparent member such as glass and a pattern (not illustrated) of a obstructing part or a light-transmitting part is provided on the plate surface thereof. A hole 23a of the rotating disk 23 is put on the thinner part 21b of the shaft 21, the bottom surface of the rotating disk 23 is seated on the step 21a, and an annular ring 24 is attached on the top surface of the rotating disk 23. 25 denotes an adhesive applied between the ring 24 and the shaft. 26 denotes a holder of a light-emitting element and the light-emitting element 27 is attached to the holder. 28 denotes a subboard attached to the housing 20 and a photodetector element 29 is attached on the plate surface of the subboard 28. 30 denotes a main board attached to the housing 20 via a supporter (not illustrated), said main board 30 and the subboard 28 are connected by means of a connection wire (not illustrated), and electronic components composing a circuit to detect the amount of rotation are mounted on the plate surface of the main board 30.
Now, the assembly work for an optical encoder will be explained.
The hole 23a of the rotating disk 23 is put on the thinner part 21b of the shaft 21 and the bottom surface of the rotating disk 23 is seated on the step 21a of the shaft 21. The ring 24 is fitted in the thinner part 21b of the shaft 21, a projecting piece of the ring 24 is bent, and the rotating disk 23 is tentatively fixed by means of the ring 24 and the step 21a of the shaft 21. In this condition, while the rotating disk 23 is rotated, the rotating disk 23 is centered. The adhesive is applied through the clearance of the projecting piece of the ring 24, the ring 24 is buried under the adhesive, and the rotating disk 23 is fixed on the shaft 21.
The rotating disk 23 is thus fixed on the shaft 21 and then, the light-emitting element 27, the photodetector 29, and the subboard 28 are attached thereto.
However, in the prior-art optical encoder, the ring 24 is buried under the adhesive so that fixation strength is provided for the rotating disk, and therefore, the adhesive enters between the hole 23a of the rotating disk 23 and the shaft 21 and between the rotating disk 23 and the step 21a of the shaft 21, this adhesive shrinks due to changes in temperature, and displacement, eccentricity, and surface oscillation of the rotating disk are caused, whereby detection accuracy is lowered.
Therefore, the present invention aims to provide a disk-mounting structure for an optical encoder which has a high detection accuracy without producing displacement, eccentricity, or surface oscillation of the rotating disk.
In order to solve the above described problem, according to the present invention, a disk-mounting structure for an optical encoder comprising a housing, a bearing attached to the housing, a shaft supported on the bearing, a rotating disk which is fixed to the shaft and has a code pattern comprising a light-transmitting part or a light-obstructing part, a photodetector which directly or indirectly receives light, from a light-emitting element, which has passed through the light-transmitting part of the rotating disk and changes the light to an electric signal, and a board mounted with a circuit for detecting the amount of rotation or rotational position of the shaft from the electric signal further comprises
a rotating disk in which a hole through which the shaft penetrates is opened at the center thereof and the rear surface thereof is seated on a step of the shaft, a spacer which is loosely fitted over the shaft and placed on an adhesive surface applied on the top surface of the rotating disk, a ring which is thrust onto the shaft for fixing the spacer and the rotating disk tentatively to the shaft, and adhesive applied on the spacer, the ring, and the shaft.
As mentioned above, according to the present invention, since the adhesive for fixing the rotating disk is prevented from entering between the outer circumference of the shaft and the inner circumference of the hole of the rotating disk or between the step and the rotating disk, the displacement, eccentricity, or surface oscillation of the rotating disk does not occur, whereby an optical encoder having a high detection accuracy of the amount of rotation and rotational position can be provided.